Barack Obama endorsed Joe Biden's White House bid on Tuesday, saying his longtime vice president can unify and "heal" a nation struggling through some of its darkest moments.

The formal backing by perhaps the most popular politician in America is the latest shot in the arm for Biden's surging candidacy, and a further sign that Democratic leaders are rallying around the party flagbearer ahead of November's election.

"Joe has the character and the experience to guide us through one of our darkest times and heal us through a long recovery," Obama said in a 12-minute video filmed at his home in Washington and released online.

"I believe Joe has all the qualities we need in a President right now," Obama said, calling his choice of Biden as running mate in 2008 "one of the best decisions I ever made."

The 77-year-old former vice president and Democratic stalwart is the party's presumptive nominee to challenge Donald Trump, after his lone remaining rival Bernie Sanders dropped out of the race last week.

The leftist US senator from Vermont endorsed his former rival on Monday, saying it was time for Americans of all political stripes to "come together" in support of Biden.

Former US president Barack Obama endorsed his vice president Joe Biden's 2020 White House bid
Former US president Barack Obama endorsed his vice president Joe Biden's 2020 White House bid AFP / MANDEL NGAN

Two-term president Obama also praised Sanders as a champion of progressive ideas, a passionate candidate whose energy and enthusiasm inspired young voters by the millions.

And he said it was time for those progressive supporters to help defeat the Republican incumbent.

"Right now, we need Americans of goodwill to unite in a great awakening against a politics that too often has been characterized by corruption, carelessness, self-dealing, disinformation, ignorance, and just plain meanness," Obama said.

"To change that, we need Americans of all political stripes to get involved in our politics and our public life like never before."

former US president Barack Obama
This screengrab shows former U.S. president Barack Obama in a 12-minute video he posted on April 14, 2020, in which he endorses his former vice president Joe Biden's White House campaign. BidenForPresident/Handout

Obama's endorsement comes as Biden and Trump have been forced off the campaign trail by the deadly coronavirus pandemic.

Under typical campaign conditions, such high-profile support would be followed by Obama's appearance at a major rally alongside Biden, generating major national buzz and prompting a deluge of campaign donations.

But it remains unclear when, if at all, on-the-ground campaigning will resume in 2020.

Obama forged a special bond with Biden during the eight years the former Delaware senator served as his vice president, awarding him the presidential medal of freedom in January 2017.

But thus far in the 2020 race the nation's first African-American leader had largely flown under the political radar, preferring Democrats battle for the nomination without his interference.

While publicly neutral, Obama did play a role in persuading Sanders to end his campaign and endorse Biden, The New York Times reported.

And despite his silence he was given a starring role in multiple campaign advertisements by Biden, Sanders and other candidates as they scrambled for advantage ahead of key statewide primaries such as those on Super Tuesday on March 3.

At campaign events and debates Biden made sure to show he is running as Obama's heir, routinely highlighting the partnership with his former boss.

But Biden had made clear from the start that he would run for president on his own terms.

"I asked president Obama not to endorse," Biden said back on April 25 on the day he launched his candidacy. "Whoever wins this nomination should win it on their own merits."

Obama's endorsement comes relatively early in the race compared to 2016.

That year he waited until June 8 to endorse Hillary Clinton, who had clinched the Democratic nomination against rival Sanders two days earlier.